This is nearly word-for-word a popular homophobic argument against gay
people, btw, in case anyone didn’t recognize it; which makes it hard to
take OP as anything other than a troll, but I’ll keep going anyway since hey, other people might learn stuff.
So…
People at large have an extremely skewed idea of what evolutionary imperatives actually are (see the ’strongest’ vs ‘best fit for environment’ confusion) at the heart of it, and even less so what constitutes a successful evolutionary strategy. You really have to stop and take a look at your understanding of popular scientific concepts to make sure they aren’t just thin blankets of “science says” thrown over common unexamined schema of how you think the world ought to work.
So point one: Evolution doesn’t design, intend, or enforce anything. It is not an active winnowing and progressing force. Literally the ONLY question that evolution poses is “is whatever you’re doing working well enough for your species to continue functioning in this environment?” Nothing else. So to start off with, the fact that the human race is doing just fine regardless of whether specific individual members aren’t doing the do means that, as a species, we’re as evolutionarily adapted as we need to be and the presence of gay or asexual people in the population is not hurting us.
Point two: Evolution is a species-level pressure, not an individual one. Evolution doesn’t care whether any given individual passes on their genes. Only that the larger population does.
And because of that, you can have all SORTS of evolutionarily successful strategies that don’t involve every member of the species directly generating more offspring. You can get evolutionarily successful strategies that look pretty fucking wild from our basic, male-and-female-combine-to-make-offspring model. You get gay male penguins and gay male swans stealing non-blood-related eggs from females to raise together and the females are just fine with this because now they can go live a swan bachelorette lifestyle without the unbelievable drain of having to raise their kids on top of the already-considerable biological drain of gestating them. You have species with three different subtypes of males and two of females that interact in byzantinely complex substructures of who fucks who and who has the babies and who raises them. You have hermaphroditic species that literally have dueling cock battles over who gets away with not being pregnant. You have species that will lay their eggs in the nests of other species to get out of child-rearing and species that will deliberately bring in individuals of other species to act as nannies.
Both child-bearing and child-rearing are incredibly resource draining activities, in nature as in here. Some species have adopted the ‘zerg rush’ method and it works well for them – spiders or fish who lay hundreds or thousands of eggs at once in hopes that at least a few of them will make it but devote essentially no resources to childrearing beyond that – but as a general rule, the more complex lifeforms get and the longer it takes to raise a baby from infancy to self-sufficiency, the greater the amount of labor and resource investment. Humans are way on the far extreme end of that particular scale. There is far more involved in human reproduction than merely just producing the babies.
So imagine you have a species of bird who raise several generations of eggs in one nest. The first egg to hatch gets designated the “babysitter,” and instead of leaving the nest once it reaches adulthood, sticks around to help its parents raise its younger siblings instead. With more bodies to collect food and defend the nest against predators, the rest of the young have a better chance of getting enough food and surviving to adulthood without getting eaten. Even if the babysitter bird never has any eggs of its own, it has still contributed materially to the advancement of its genetic line, since each of its siblings shares the same component of genes (50%) as any child of its own would.
That is just one, hypothetical model to show how homosexual and asexual individuals in a disexual species would nevertheless act as part of a successful reproductive strategy. For a less-hypothetical model, simply look around you. The more complex the society, and the more complex the requirements to raise an individual offspring to adulthood, the more complex and vital these indirect contributions become. We all work to create an environment where children survive and flourish, where they don’t risk being eaten by bears or starving through a hard winter. (Sometimes one can doubt whether we succeed in that aim, but… well, we’re at least trying.)
We are, asexuals and homosexuals and allosexuals alike, all equal contributors to the survival and perpetuation of our species, even if we never directly touch a baby in our lives. Anyone who says otherwise is a fool, or a malicious liar.